function dVaa = Vab(dAo, dBo, dCo, dAc, dBc, dCc)
dA = dAc-dAo;
dB = dBc-dBo;
dC = dCc-dCo; 
dR = sqrt(dA^2+dB^2+dC^2);
dVaa = (3*dA*dB)/dR^5;
end


